I was trying to compare views of M42 using various binocular options and found it curious that in some views the outline of the nebula were shown but not in others, and displays seemed counter-intuitive. I haven't done much binocular viewing, and the setup in SkyTools for binoculars seems trivial, so curious about the results shown below.

It is very difficult to say without knowing all of the details, meaning everything from the specific binocular data to how much light pollution is selected to how high M42 was in the sky.

In general, an object is displayed in the eyepiece view when it is deemed to be detectable to the eye. It is not displayed when it is not detectable. If it can be seen depends on many factors, including: exit pupil, magnification, light pollution, he pupil diameter of the observer, how high the object is in the sky, twilight, and moonlight.

Another possibility is that, since each of these binoculars has its own custom charts, diffuse nebulae may simply be turned off on some of those charts.

My advice is to carefully set up your location and observer as well as the binoculars. Make sure that diffuse nebulae are enabled in the View Controls, and that the chart preferences for "Rendered Diffuse Nebulae" are set to colors that are visible against the chart background color. Then make sure that you are comparing under the exact same conditions, e.g the same location and the same time. Ensure that there is no twilight or moonlight and that M42 is high in the sky.

Thanks Greg. I thought maybe there was a simple explanation such as aperture to magnification ratio or something similar, but it sound like it could b any number of factors. I will play with it a bit and see. Thanks for the quick response.